Pressure has a lot to do with cleanliness of the burn, too low and it will be dirty..
Apparently this is quite true.
The 7mm magnum book starting load using Ramshot Magnum & the 139 grain Hornady is one of the filthiest loads I've ever had to clean up after. It sure shoots well though, my last three shot group at 100 yards out of a cold barrel was a perfect triangle @ .50"
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